2 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
a single State, lichen species in general are so widely distributed that 
the descriptions and figures will, it is hoped, prove helpful in the 
determination of lichens throughout North America, and especially 
throughout the northern United States and British America. 
As the method of determining the application of generic names of 
lichens is still in controversy, it has seemed best to use them in the 
traditional sense, stating the types of the genera according to the 
‘first species” rule. In instances in which the type falls without the 
genus as at present understood, statement is made to that effect. It 
is manifest that, until there is general agreement as to a method of 
typifying genera and until all the nominal lichen genera have been 
typified according to this method, anything done toward a revision 
of the generic names would as likely as not have to be done over. 
Likewise, until lichen species have been typified as exhaustively as 
possible, any extended synonymy of them can be of little value. Con- 
sequently, nothing has been attempted in the way of synonymy of 
the species described, beyond stating, in instances in which a species 
has been transferred from the genus in which it was first. described, 
the citation for the first combination as well as for the one used herein. 
The priority of specific names can be settled only by the examination 
of a large number of type specimens in European herbaria, and as the 
author has not been able to see these, there is no doubt that at least 
a small portion of the specific names here used will eventually have 
to give way to others. 
Until we know more regarding the phylogeny of lichens and the 
structure of sexual reproductive tracts, anything approaching 
finality in their taxonomy is scarcely possible. The writer has been 
influenced in classification by a number of authors, especially by 
A. Zahlbruckner in Engler and Prantl, but holds himself alone respon- 
sible for the arrangement of families and genera used. In fixing 
upon the sequence of genera and of closely related families, both spore 
characters and vegetative structure have been taken into account. 
Doubtless the concurrent resemblance in these two respects is often 
merely accidental and it does not therefore always imply the close 
relationship that juxtaposition would indicate. These difficulties 
arising from this circumstance can only be settled by future investi- 
gations of anatomy, reproductive processes, and phylogeny. 
It need scarcely be stated that the descriptive catalogue is the 
main feature of this work and that the preceding chapters, treating 
very briefly of the origin and nature, the morphology, the repro- 
ductive processes, and the uses of lichens, are intended as an aid in 
using it. It has, therefore, not been thought necessary to add a list 
of references to this part of the lichen literature. 
The writer takes this opportunity to express his thanks to several 
persons who have aided in the work in various ways. First of all, he 
